A common approach to field emission uses an atomically sharp tip that concentrates electric field lines of an applied potential, thus effecting local geometric field enhancement at the cathode-vacuum interface. The enhanced field facilitates electrons' tunneling into vacuum through the steep energetic barrier at the emission surface. Arrays of geometric field enhancement cathodes generally require an applied potential of 20 to 200 V to obtain practical working current densities, i.e. greater than 1 mA/cm2.
With reference to FIG. 1, another approach uses two electrodes 100, 100′ placed on an insulating substrate 104, such as PYREX glass. When sufficient bias is placed across these electrodes 100, 100′, electrons are emitted into the vacuum. The cathode efficiency, i.e., the ratio of the emitted current to the current flowing between the electrodes, for such an arrangement ranges from 1×10−4 to 1×10−2. The conduction band edge of such a material is higher than the vacuum energy level, so emission of conduction band electrons to vacuum from the bulk of the cathode body is energetically favored. However, in general there is a significant barrier to injection of electrons into the cathode material from a metal contact. If the semiconductor forms a Schottky diode on metals, then when an electric field is applied across the semiconductor, the dopant impurities become positively ionized and form a depletion region at the metal-semiconductor junction, giving rise to a local field enhancement. When nitrogen-doped diamond or Cs2Si4O9 glass is used as the semiconductor instead of PYREX glass, the electric field at this junction is often greater than 107 V/cm, sufficient to cause tunneling of electrons from the metal into the semiconductor and markedly increasing the cathode efficiency. Once the electrons are in the NEA semiconductor they can be emitted directly into vacuum.
Although excellent electron emission characteristics have been observed from known emitter structures, especially those incorporating cathode bodies of diamond and amorphous diamond-like materials, practical applications of these cathodes remain limited by the fabrication techniques required to achieve their geometry, bias currents that are often substantially larger than the emitted current, and an operating environment that typically requires a vacuum of less than 10−9 Torr. Accordingly, there is a need for surface-emitting cathodes that are inexpensive to fabricate, have emission currents that scale with area, and can operate with no gate bias voltage in a compromised vacuum.